The inventive concepts described herein relate to semiconductor memory devices, and more particularly, the inventive concepts relate to memory systems including nonvolatile memory and to read reclaim methods executed by memory systems including nonvolatile memory.
Semiconductor memories constitute a vital microelectronic component of digital logic system design, such as computers and microprocessor-based applications ranging from satellites to consumer electronics. Advances in the fabrication of semiconductor memories including process enhancements and technology developments through scaling for higher densities and faster operating speeds help establish performance standards for other digital logic families.
One category of semiconductor memory is volatile random access memory (RAM). In volatile RAM devices, logic information is typically stored either by setting up the logic state of a bistable flip-flop such as in a static random access memory (SRAM), or through the charging of a capacitor as in a dynamic random access memory (DRAM). In either case, the devices are considered volatile in that stored data is lost if the supply of power thereto is interrupted.
Another category of semiconductor memory is non-volatile memory which retains stored data even when the supply of power is interrupted. Examples include Mask Read-Only Memory (MROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), and Electrically Erasable Programmable Read-Only Memory (EEPROM). The data stored in non-volatile memory may be permanent or reprogrammable, depending upon the fabrication technology used. In addition, non-volatile memories may be used for program and microcode storage in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries. A combination of single-chip volatile and non-volatile memory storage modes is also available in devices such as non-volatile SRAM (nvSRAM) for use in systems that require fast, programmable non-volatile memory. In addition, dozens of special memory architectures have evolved which contain additional logic circuitry to optimize their performance for application-specific tasks.
Among the previous given examples of non-volatile memory, MROM, PROM, and EPROM are not capable of being erased and then written to by a corresponding system itself, and it is thus difficult or impossible for general users to update stored contents. On the other hand, EEPROM is capable of being electrically erased and then written at a system level. Applications of EEPROM have therefore expanded to auxiliary memory and system programming where continuous updates are needed. An example of this is the widely adopted flash memory.